The process of fabricating high strength materials from powders such as ceramic and metal powders generally involves preparing “green” materials that include the powder and a thermoplastic binder of variable composition. As part of the fabrication process, the binder typically is removed from the material in a binder burnout step and the powder consolidated and densified in order to obtain a final structure having the desired properties, including strength and hardness. Methods of consolidation and densification include sintering processes such as, uniaxial hot pressing, hot isostatic pressing, overpressure sintering and atmospheric (pressureless) sintering.
Sintering processes, are critical in the fabrication of materials from ceramic and metal powders. Equipment used in pressure sintering processes including hot isostatic pressing (HIP) and uniaxial hot pressing must be designed to accommodate the high temperatures and high pressures associated with these sintering methods. Purchase, operation and maintenance costs for the HIP and uniaxial hot press equipment may be high as a result of the need to incorporate vessels capable of withstanding high pressures or hydraulic controlled rams into their respective designs. There are also additional costs in addressing safety requirements and designs for the safe and reliable operation of high pressure equipment. Additionally, the capacity of HIP and uniaxial hot press equipment is limited by these requirements. Thus, production volume capabilities are reduced, which further increases production costs. Furthermore, pressing is generally limited and cannot be used effectively with three-dimensional structures having more complex geometries.
Pressureless sintering furnaces generally are less expensive to purchase, operate and maintain as compared to equipment for pressure sintering. They also provide larger production volume capabilities and lower overall production costs. However, an important disadvantage associated with pressureless sintering is the potential inability to achieve effective sintering of a material in the absence of pressure.
Fibrous monoliths (FMs) are a unique class of structural ceramics. FMs are monolithic ceramics that are manufactured by powder processing techniques using inexpensive raw materials. Methods of preparing FM filaments are known. U.S. Pat. No. 5,645,781 describes methods of preparing FM composites by extrusion of filaments having controlled texture. As a result of the combination of relatively low costs of manufacture and benefits of enhanced materials performance, FMs have been used in a wider range of applications than heretofore typical for ceramics. Fibrous monoliths typically have been formed to various fibrous textures. For example, FM filaments have been woven into thin, planar structures. Alternatively, the filaments have been formed into three-dimensional structures having complex geometries.
Generally, the macroarchitecture of an FM composite includes a plurality of filaments each including a primary phase in the form of elongated polycrystalline cells surrounded by at least a thin secondary phase in the form of a cell boundary. The material selected for the cell phase differs from the material selected for the cell boundary phase in type and/or composition. Thus, the various materials comprising a FM composite each have different material properties.
This “multi-phase” nature of FM composites, along with the possibility that the composites are formed into complex structures, can increase the difficulties encountered when attempting to sinter such composites. Significantly, when two or more materials are used and are to be maintained essentially separate from each other in a composite component, the ability to effectively sinter the FM composite component can be severely limited or even prevented. Because the material properties of the two phases differ, the range of physical and chemical conditions that lead to effective sintering of the composite can be restricted. The difficulty in identifying an effective sintering regime increases further as additional materials are included in the composite. Moreover, the potential for unfavorable interactions between materials that can limit sinterability increases as additional materials are added to the composite.
There remains a need for more efficient, cost-effective sintering processes that can be utilized during fabrication of fibrous monolith composite structures, particularly those having complex geometries.